To preserve sources of raw materials and to reduce the amount of plastics materials requiring disposal, numerous recycling process are being investigated. Thus it has long been common practice in the manufacture of articles from thermoplastic materials by injection moulding to add a certain percentage of recycled material, generally of the same kind, without any marked reduction in the quality of the material in comparison to the virgin product.
While thermoplastic materials are very suitable for use in such processes, thermosetting plastics (duroplastics) and duromers are not. Because of their chemical structure it is not at present thought to be possible to recycle these products and return them to a material circuit in the same way as thermoplastic materials.
Thermosetting plastics are synthetic materials that are, as a rule, amorphous and retain their strength up to their decomposition temperature. The chemical reaction which is usually brought about by heating leads to irreversible cross-linking. At higher temperatures the thermosetting plastics decompose.
Moreover the duromers and the polyurethane soft foams are by definition fully reacted and are therefore generally not thought to be susceptible to further processing. Consequently it has not hitherto been considered possible to carry out economical recycling, particularly for direct reuse in the form of sound insulating bodies.
Some complex and expensive recycling processes have however been described by Brandt: Kunststoff-Journal 9 (1975), No. 5, pp. 22-24. To the comminuted-to-pulverised waste foam, for example from cutting up foam slabs, a solvent consisting of dialkanolamine in glycol heated to 185.degree.-210.degree. is added and the mixture is allowed to stand for about 20 hours. The chemical reactions that occur when this is done yield regenerated polyols in a very high percentage yield based on the total weight of raw material. This procedure, however, is not practicable on economic grounds.
The "cracking" of foams by hydrolysis, in which OH-groups are caused to combine with other molecules by temperature and pressure, is also known. The resulting mixture of polyols is not, however, the same as the polyols that are used as starting material for foam manufacture.
Processes are also known by means of which cut foams consisting, for example, of polyether-polyols and isocyanates, can be thermally pressed to shaped articles. These are applied mostly to pieces that have been cut out from foamed slab material.